You are here

normalized vegetation index

Abstract:

This database contains mean NDVI values for 200 m diameter circles encompassing Rodent Webs on the Sevilleta National Wildlife Refuge (NWR), for 21 Landsat TM scene dating from 1984 to 1993. These NDVI vectors were generated as part of cooperative project between the Sevilleta LTER and the Indian Health Service, to study the 1992 Hantavirus outbreak.

Rodent Web coordinates were extracted from the SIMS GPS Master Archive database, converted into an Arc/Info point coverage, then buffered to create circular polygons of 200 m diameter centered on the Web center stake. Then Arc/Info base and GRID routines were used to overlay the Web polygon coverage on each NDVI image, and mean NDVI values for each 200 m polygon were calculated; this generates an NDVI vector at each of the Web locations. The exact procedure log is included in the Additional Information below.

Additional information:

Additional Study Area Information

The Sevilleta National Wildlife Refuge (NWR) is located in Socorro County, New Mexico, in the United States of America. The Sevilleta LTER was initiated as the Sevilleta National Wildlife Refuge, a former Spanish land grant now administered by the U.S. Fish and Wildlife Service. The LTER recently has been expanded to a research area of approximately 3,600 km2 that ranges from Rio Grande riparian forests ('bosque') and Chihuahuan Desert up to subalpine forests and meadows. Four dedicated research areas comprise the core sites; Sevilleta National Wildlife Refuge (100,000 ha), Bosque del Apache National Wildlife Refuge (25,300 ha), Sierra Ladrones Wilderness Study Area (28,390 ha) and the Magdalena Mountains Research Area (15,000 ha). The research region spans the Rio Grande basin with elevations ranging from 1,350 m at the Rio Grande to 2,195 m in the Los Pinos Mountains in the east, to 2,797 m at Ladrone Peak in the northwest, and to 3,450 m in the Magdalena Mountains to the southwest.

Climate is characterized by an intriguing combination of abundant sunshine, low humidity and high variability in most factors. The site exists in the boundary between several major air mass zones which contributes to the dynamics of the local climate. Precipitation ranges from <100 mm to 600 mm with an average of 280 mm. Summer precipitation occurs as intense thunderstorms often accounting for over 1/2 of the annual moisture. El Nino and La Nina events influence winter precipitation and marked variations occur on an inter-annual basis. Mean monthly temperatures range from -2.5 C to 27 C. Topography, geology, soils, and hydrology, interacting with major air mass dynamics, provide a spatial and temporal template that has resulted in the region being a transition zone for a number of biomes.

The region contains communities representative of, and at the intersection of, Great Plains Grassland, Great Basin Shrub-steppe, Chihuahuan Desert, Interior Chaparral, and Montane Coniferous Forest. The elevational gradient of the Magdalena Mountains provides further transitions for Interior Chaparral, Pinyon-Juniper Woodland, Petran Montane Conifer Forest, Petran Subalpine Conifer Forest, and Subalpine Grassland. The regional location at the junction of a number of biomes is critical for quantifying (1) gradient relationships with distance, (2) the scale-dependent or independent nature of spatial variability, (3) how steep gradients influence system properties, (4) integrated responses across the region, and (5) biome responses to climate change. Many species of these communities are at their distributional limits. For example, 54 plant species terminate their distributions within the Sevilleta and some represent major life forms and physiologies, such as the C3 perennial grasses. Reptiles provide a dramatic example as 47 of the 58 species end their distributions in the vicinity of the Sevilleta (33 are northern limits of desert species). An important feature of the biodiversity of this region is the number of examples of sympatric swarms of closely related species. This sympatry affords opportunities for studying the evolutionary differential of species.

Well, I've tried several different approaches to developing correction factors for the NDVI values at the Web locations on the Sev, but have been foiled everytime. I am convinced that we'll just have to go back to the original bands and perform atmospheric correction on them utilizing the NASA correction algorithms Eric Vermote is developing. Unfortunately, that's probably a month or two away. In the meantime, I'm not sure what to think of the Landsat 4 imagery (6/4/92 and 7/6/92 scenes) since they seem to show unusually high NDVI values in relation to the Landsat 5 scenes just before and just after those dates (4/9/92 and 7/14/92). I know we received a lot of rain in May 1992, so some of this could be real but not sure how much. In addition, I've noticed that NDVI values generally seem higher starting in 1992, then before; and this could be due to a change in radiometric calibration procedures that EOSAT implemented in 1992. This points out the difficulty with doing multi-temporal analyses (looking for absolute changes at a location) with TM imagery. The next generation sensors (e.g., MODIS) will eliminate much of these variations and unknowns but that doesn't help us now.

Anyway, I've also taken a look at each image for cloud and cloud shadow effects, and have the following list of Webs and dates that should be excluded: